Accurate assessment of oxygen saturation of time varying hemoglobin compartments such as arteries is fundamental to the support of critical-care medicine. Tissue saturation information is also important. Lack of immediate and/or continuous information can lead to potential diagnostic errors, particularly in critically ill patients in whom life-threatening changes can occur rapidly. Arterial compartment saturation measurements are important indicators of vascular oxygen supply. In addition, tissue saturation measurements are important indicators especially during periods of low blood pressure or no pulse.
Various diagnostic measurements are available to assist doctors in determining a patient's oxygenation status, and to protect the patient against dangerous low blood oxygen conditions. Traditionally, blood gases are measured by invasive sampling, either through an indwelling arterial catheter or by arterial puncture. Such sampling, however, can cause significant blood loss, especially for infants. Moreover, since sampled blood is later analyzed in a laboratory, blood gas values are only available after considerable delay.
Beginning in the early 1930's, considerable efforts were made to develop non-invasive optical techniques for accurate measurements of oxygen in the blood. Due to these efforts, the first non-invasive oximeter was developed. Thereafter, known pulse oximetry was introduced in about 1974 and entered clinical practice in the 1980's. It continues to develop, and today pulse oximetry is widely acknowledged as one of the most important technological advances in patient monitoring. An important advantage of pulse oximeters is the capability to provide continuous, safe, and effective monitoring of arterial blood oxygenation non-invasively at a patient's bedside.
Pulse oximetry is based on the physical principle that oxygenated and deoxygenated hemoglobin show different absorption spectra. Deoxygenated hemoglobin absorbs more light in the red band (typically 600 to 750 nm), i.e., it looks less red, and oxygenated hemoglobin absorbs more light in the infrared band (typically 850 to 1000 nm). Thus, known pulse oximeters use one wavelength in the red (typically 660 nm), and one wavelength in the near-infrared (typically 940 nm) spectral range to measure the oxygen saturation of arterial blood. The ratio of light absorbencies at the two wavelengths correlates with a proportion of oxygenated to deoxygenated hemoglobin in tissue, for example, the brain, capillaries, veins, and skeletal muscle. Of all the light absorbed, however, only that absorbed by the pulsating parts of tissue correlates to arterial O.sub.2 saturation.
A problem exists with known pulse oximetry since an arterial saturation reading is given on the basis of an empirical scaling. The empirical scaling is based on a preliminary calibration using a population of reference subjects. Due to a lack of physical models, the empirical relationship between a ratio of the pulsatile absorbencies and the arterial saturation is obtained from a large group of healthy volunteers. This empirical scaling relates detected signals to the optical absorption of hemoglobin. Additionally, empirical scaling is accomplished by the use of a continuous wave light which does not afford discrimination of the absorption and the scattering contributions due to light attenuation caused by surrounding tissue. Such empirical scaling has a high potential for error at low saturation (&lt;80%) since manufacturers cannot induce severe hypoxia in volunteers for calibration purposes, and at high saturation (&gt;97%) limits.
Another drawback of conventional pulse oximeters is the requirement to have sensors on either side of an intervening body part to take a reading. Arterial saturation measured in a finger or a toe may not be representative of the systemic arterial saturation (SaO.sub.2). Known pulse oximeters work in transmission geometry so that light is transmitted through tissue from one side of a body part, such as a finger, and measured by a photodetector at the other side of the body part. Obviously, such pulse oximeters can only be applied to relatively small body parts such as a finger, toe, earlobe, or nose, but saturation measurements at the brain, for example, are highly desirable.
Additionally, such small body parts only contain peripheral circulation, and local saturation measurements at the periphery may not be indicative of systemic saturation. In infants, for example, the presence of pulmonary hypertension and shunting through a patent ductus arteriosus can make a saturation amount in the foot significantly different from a saturation amount in the head. Furthermore, while changes in pulmonary function can be detected in the ear in a few seconds, it takes as long as 30 seconds to detect in the finger or foot. For these reasons, it is desirable to obtain real time measurements and to measure the local arterial saturation at tissue of interest, typically the brain.
Recently, the introduction of time-resolved optical spectroscopy in conjunction with diffusion theory has lead to quantitative tissue spectroscopy, as described in commonly owned U.S. Pat. No. 5,497,769 to Gratton et al., which is incorporated by reference herein. Known optical tissue oximetry measures the hemoglobin saturation in tissue (y) and is most sensitive to the blood in the capillaries where the oxygen exchange with tissue occurs.
However, known frequency-domain tissue spectroscopy only measures tissue saturation to give indications of tissue oxygen consumption, and do not measure time-varying hemoglobin compartment saturation. Measuring time-varying hemoglobin compartment saturation, such as arterial saturation, is important since such hemoglobin compartment saturation gives indications of vascular oxygen supply, as discussed above. There is a need for time-resolved measurements to provide readings for time-varying hemoglobin compartment saturation, as well as tissue saturation, to yield a balance between a local oxygen supply and oxygen consumption.
Thus, there is a need for an improved method which addresses some or all of the aforementioned drawbacks. A new method should overcome the limitations of known pulse oximetry such as its dependence on an empirical table. Moreover, an improved method should work with arbitrary and desirable tissue locations.